Sun Path - Effect of The Tilt of The Earth

Effect of The Tilt of The Earth

Sun path at any latitude and any time of the year can be easily determined as they follow a simple geometry, as described here.

Earth's rotation tilts about 23.5 degrees on its pole-to-pole axis, relative to the plane of Earth's solar system orbit around the Sun. As the Earth orbits the Sun, this creates the 47-degree peak solar altitude angle difference, and the hemisphere-specific difference between summer and winter.

In the northern hemisphere, the winter sun rises in the southeast, peaks out at a low angle above the southern horizon, and then sets in the southwest. It is on the south (equator) side of the house all day long. Vertical south-facing (equator side) glass is excellent for capturing solar thermal energy.

In the northern hemisphere in summer (June, July, August), the Sun rises in the northeast, peaks out nearly straight overhead (depending on latitude), and then sets in the northwest. A simple latitude-dependent equator-side overhang can easily be designed to block 100% of the direct solar gain from entering vertical equator-facing windows on the hottest days of the year. Roll-down exterior shade screens, interior translucent-or-opaque window quilts, drapes, shutters, movable trellises, etc. can be used for hourly, daily or seasonal sun and heat transfer control (without any active electrical air conditioning).

The latitude (and hemisphere)-specific solar path differences are critical to effective passive solar building design. They are essential data for optimal window and overhang seasonal design. Solar designers must know the precise solar path angles for each location they design for, and how they compare to place-based seasonal heating and cooling requirements.

In the U.S., the precise location-specific altitude-and-azimuth seasonal solar path numbers are available from NOAA – the "equator side" of a building is south in the northern hemisphere, and north in the southern hemisphere, where the peak summer solstice solar altitude occurs on December 21. The sun rises roughly in the east and sets in the west everywhere on Earth, except in high latitudes in summer- and winter-time.

On the Equator, the Sun will be straight overhead and a vertical stick will cast no shadow at noon solar time on March 21 and September 22 (the equinoxes). More over, on Vernal Equinox, the points beyond (northwards) and below (southwards) the subsolar point (on equator) the shadow of this vertical stick will point towards a direction little westwards of true north (NNW) reading 336.5° from true north and little eastward of true south (SSE) reading 156.5° from true north severally and on Autumnal Equinox, the points beyond (northwards) and below (southwards) the subsolar point (on equator), shadow will point towards a direction little eastwards of true north (NNE) reading 23.5° from true north and little westward of true south (SSW) reading 203.5° from true north severally, the same stick will cast no shadow on June 21, the summer solstice in the northern hemisphere when Sub solar point is on Tropic of Cancer 23.5° north of equator, although on points beyond (northwards) 23.5° north the shadow will point towards true north and points below (southwards) 23.5° north the shadow will point towards true south. Same will take place on December 21, the winter solstice for the northern hemisphere, when subsolar point will be on Tropic of Capricorn 23.5° south of equator casting no shadow over there, however on points beyond (northwards) Tropic of Capricorn solar noon shadow will point towards true north and points below (southwards) Tropic of Capricorn the shadow will point towards true south. Concisely, the solar time noon shadows of objects on points beyond and below subsolar points will point towards true north and true south respectively only when the solar declination has its maximum positive (δ☉ = +23°26') or maximum negative (δ☉ = −23°26') value, unlike on Equinoxes when sun is neither declined to north nor to south (δ☉ = 0°) and solar time noon shadows points towards NNW and NNE for north of equator but SSE and SSW for south of equator on Vernal & Autumnal Equinoxes, respectively. For north of the Arctic circle, and south of the Antarctic circle, there will be at least one day a year when the sun is not above the horizon for 24 hours, and at least one day (six months later) when the sun is above the horizon for 24 hours.

In the moderate latitudes (between the circles and tropics, where most humans live), the length of the day, solar altitude and azimuth vary from one day to the next, and from season to season. The difference between the length of a long summer day and a short winter day increases as one moves farther away from the equator.